INFLUENCE OF PHOSPHATE ADSORPTION ON LEAD IMMOBILIZATION AT THE GOETHITE-WATER INTERFACE

Adsorption to the surfaces of iron oxides and other reactive minerals can influence the transport of lead in contaminated soils and groundwater. The presence of phosphate can strongly influence the immobilization of lead through either the formation of ternary surface complexes or the nucleation of lead phosphate precipitates. The mechanism of lead immobilization can affect the long-term stability of immobilized lead and the subsequent rates of remobilization.

The influence of phosphate on lead immobilization was experimentally investigated using goethite-coated quartz sand as a model porous media with reactive iron oxide surface coatings. In single sorbate experiments, the adsorption of either lead or phosphate to goethite-coated sand was successfully modeled using slight modifications of surface complexation models developed for adsorption to pure goethite. Phosphate adsorption was modeled by considering only adsorption to the goethite component. Lead adsorption was dominated by adsorption to the goethite, but additional reactions for lead adsorption to the exposed quartz surface were necessary. In binary sorbate experiments, reactions involving both phosphate and lead were necessary for interpreting lead immobilization. At relatively low phosphate concentrations, the presence of phosphate increased lead adsorption in the acidic pH range through the formation of lead-phosphate-goethite surface complexes. For solution conditions that were supersaturated with respect to the lead phosphate mineral chloropyromorphite (Pb₅(PO₄)₃Cl_(s)), the presence of phosphate resulted in chloropyromorphite precipitation at the solid-water interface. The influence of reaction time on lead uptake was examined from one day to one month. Reaction time had the most significant influence on lead uptake for experimental systems that were closest to chloropyromorphite saturation.